The invention relates to a multi-stage compression refrigeration apparatus having a multiplicity of compression means for compressing a refrigerant in multi-stages.
A typical multi-stage compression refrigeration apparatus for use in a refrigerator and an air conditioner includes a rotary compressor consisting of a first and a second stage compression means which are housed in an enclosed container and each have a roller for compressing a refrigerant in the respective cylinder. The compressor performs compression of the refrigerant in two stages, first by the first stage compression means serving as a low-pressure compressor and then by the second stage compression means serving as a high-pressure compressor adapted to further compress the refrigerant gas compressed by the first stage low-pressure compressor.
Such a multi-stage compression refrigeration apparatus can attain a high compression ratio while suppressing variations of torque per one compression.
However, such multi-stage compressor has a drawback in that when a refrigerant has a high specific heat ratio, the second stage compression means has a low suction efficiency because it receives hot refrigerant heated by the first stage compression means. The multi-stage compressor also suffers from a further disadvantage that the temperature of the refrigerant is heated in the second stage high-pressure compression means to a great extent that the lubricant used therein will be thermally hydrolyzed into acids and alcohol, particularly when ester oil (for example, polyol ester, POE) is used. These acids disadvantageously develop sludges which tend to clog capillary tubes of the compressor, degrade the lubricant, and hence lower the performance of the apparatus.
In order to circumvent these problems, some compressors are provided with a cooling unit for cooling the refrigerant gas discharged from the first stage compression means before it is supplied to the second stage high-pressure compression means, thereby sufficiently lowering the temperature of the refrigerant gas discharged from the second stage compressor. For example, one type of such multi-stage compression refrigeration apparatus as shown in FIG. 5 has: a multi-stage compressor 511 which consists of a first stage low-pressure compression means and a second stage high-pressure compression means; a condenser 512; a first decompression means 513, an intercooler 514, a second decompression means 515, and an evaporator 516. The refrigerant exiting the condenser 512 is diverted into two parts, with one part passed to the intercooler 514 via the first decompression means 513, but the other part passed directly to the intercooler 514, and then passed to the second decompression means 515 and the evaporator 516. The two parts undergo heat exchange in the intercooler 514. The refrigerant exiting the evaporator 516 is fed to the first stage compression means of the multi-stage compressor 511. On the other hand, the part of the refrigerant that has passed through the intercooler 514 is mixed with the refrigerant discharged from the first stage low-pressure compression means before entering the second stage compression means.
Thus, this multi-stage compression refrigeration apparatus has a refrigeration cycle as depicted in the P-h diagram (solid line) shown in FIG. 6. In this conventional apparatus, the enthalpy of the refrigerant is reduced by xcex4Ho, as shown in FIG. 6, by the heat exchange between the two parts of the refrigerant in the intercooler 514, i.e. heat exchange between the refrigerant passed through the first decompression means 513 and the refrigerant passed directly to the intercooler 514. This arrangement may increase an enthalpy difference across the evaporator 516.
However, such conventional apparatus fails to cool the refrigerant in the intercooler 514 sufficiently prior to decompression by the second decompression means 515 due to the sensible heat in the tubes of the intercooler 514 for example, so that, at an early stage of a start-up operation, the evaporator 516 cannot create intended enthalpy difference xcex4Ho required for a normal operation (as indicated in FIG. 6).
Another drawback pertinent to the prior art apparatus is that, following stopping of the refrigeration operation, hot refrigerant in the condenser 512 flows into the evaporator 516 via the second decompression means 515, resulting in a large amount of liquefied refrigerant staying in the evaporator 516. Hence, it takes a fairly long time to have the entire liquefied refrigerant in the evaporator 516 to be evaporated during a restart of the compressor 511, thereby requiring a time for the apparatus to recover its normal operating condition. This lowers the efficiency of the apparatus.
As a measure to circumvent this problem, an integral valve system might be provided which has cooperative first and second valves mounted upstream and downstream ends, respectively, of the evaporator 516, such that the first valve is closed in response to a backflow from the compressor 511 following the stopping of the compressor and the second valve is then closed in response to the first valve, thereby stopping the backflow from the second decompression means 515 to the evaporator 516.
In this arrangement, the backflow of the liquid refrigerant into the evaporator 516 can be prevented. However, in an apparatus as mentioned above where the refrigerant discharged from the first stage compression means is mixed with the refrigerant from the condenser 512 before it is fed to the second refrigeration means, hot liquid refrigerant remaining in the condenser 512 will flow into the intercooler 514 after the compressor 511 is stopped. As a result, when the apparatus is restarted, sensible heat that remains in the intercooler 514 will prevent sufficient cooling of the refrigerant in the intercooler 514 before passing it to the second decompression means 515. Consequently, super-cooling of the refrigerant for the intended enthalpy difference xcex4Ho is not obtained by the evaporator 516.
It is therefore an object of the invention to overcome the problems as mentioned above by providing an improved multi-stage compression refrigeration apparatus having a first and a second stage compression means and equipped with an intercooler which is adapted to cool the compressed refrigerant gas discharged from the first (low-pressure) compression means. Thus, the apparatus is capable of lowering the temperature of the gas discharged from the second (high-pressure) compression means to create a large enthalpy difference in an evaporator during an early stage of startup.
It is another object of the invention to provide an improved multi-stage compression refrigeration apparatus adapted to stop the backflow of refrigerant into the evaporator and the intercooler when the apparatus is stopped, thereby allowing the apparatus to resume creation of a large enthalpy and attain an improved refrigeration efficiency during an early stage of startup.
In accordance with one embodiment of the invention, there is provided a multi-stage compression refrigeration apparatus including a compressor having a first stage low-pressure compression means and a second stage high-pressure compression means, a condenser, a first decompression means, a first intercooler, a second decompression means, and an evaporator, wherein the refrigerant discharged from the second stage compression means is passed through the condenser, and is diverted into first and second parts, with the first part passed to the first intercooler via the first decompression means, while the second part is passed to the first intercooler to undergo heat exchange therein with the first part, and then passed to the second decompression means, the evaporator, and-further to the first stage low-pressure compression means; and wherein the first part of the refrigerant exiting the first intercooler is mixed with the second part of the refrigerant discharged from the first stage low-pressure compression means before they are fed to the second stage high-pressure compression means of the multi-stage compression refrigeration apparatus, the apparatus further comprising
a second intercooler mounted downstream of the evaporator, adapted to perform heat exchange between the refrigerant that has passed the evaporator and the second part of the refrigerant before entering the evaporator.
This arrangement may sufficiently lower the temperature of the refrigerant gas discharged from the second stage compression means, and create a larger enthalpy difference in the evaporator than conventional apparatuses during an early stage of startup.
The refrigeration apparatus may be further provided with a third intercooler mounted downstream of the condenser for performing heat exchange between the refrigerant discharged from the condenser and the refrigerant discharged from the first intercooler before the latter refrigerant is mixed with the refrigerant exiting the first stage compression means, thereby feeding the mixed refrigerant to the second stage compression means. This arrangement ensures further improvement of efficiency of the apparatus.
The refrigeration apparatus may be provided with a third decompression means for decompressing the second part of the diverted refrigerant after the refrigerant has undergone the heat exchange in the second intercooler. The temperature of the refrigerant entering the evaporator is further lowered in this arrangement.
In accordance with another embodiment of the invention, there is provided a multi-stage compression refrigeration apparatus including a compressor having a first stage low-pressure compression means and a second stage high-pressure compression means, a condenser, a first decompression means, a first intercooler, a second decompression means, and an evaporator, wherein the refrigerant discharged from the second stage compression means is passed through the condenser, and is diverted into a first and a second parts, with the first part passed to the first intercooler via the first decompression means while the second part is passed to the first intercooler to undergo heat exchange therein with the first part, and then passed to the second decompression means, the evaporator, and then to the first stage low-pressure compression means; and wherein the first part of the refrigerant exiting the first intercooler is mixed with the second part of the refrigerant discharged from the first stage low-pressure compression means before they are fed to the second stage high-pressure compression means of the multi-stage compression refrigeration apparatus, the apparatus further comprising:
a first valve mechanism which is mounted upstream of the first stage compression means and adapted to be fully closed in response to a predetermined amount of backflow of refrigerant from the first stage compression means towards the evaporator;
a second valve mechanism which is mounted upstream of the evaporator and adapted to be opened/closed in cooperation with the first valve mechanism; and a third valve mechanism mounted downstream of the condenser, adapted to be opened/closed in cooperation with the first valve mechanism.
In this arrangement, should a backflow of the refrigerant gas in the first valve mechanism take place following stopping the compressor, the backflow into the evaporator and the first intercooler will be prevented by the respective second and third valve mechanisms, since they are fully closed upon occurrence of the backflow in the first valve mechanism.
The refrigeration apparatus may be further provided with a fourth valve mechanism which is mounted upstream of the first decompression means and adapted to be opened/closed in cooperation with the first valve mechanism, thereby preventing a backflow of the liquid refrigerant remaining in the refrigerant lines into the first intercooler following stopping the compressor.
The compressor may be of a multi-stage compression rotary compressor having a multi-stage compression mechanism which includes:
an electric motor contained in an enclosed container;
a rotary compression unit having a first stage low-pressure compression element and a second stage high-pressure compression element, both elements operatively coupled to the drive shaft of the electric motor element; and
a communicating tube for connecting the discharge port of the first stage low-pressure compression element and the inlet port of the second stage high-pressure compression element.
The compressor may be adapted to run in a reverse direction for a predetermined period of time before the compressor is stopped. In this arrangement, the refrigerant gas can be immediately flown from the outlet port of the compressor back to the first valve mechanism following the stopping operation.
The second, third, and fourth valve mechanisms may be constructed integral with the first valve mechanism.
The second decompression means may be a capillary tube, and the second valve mechanism may be connected to the inlet of the capillary tube. This arrangement enables down-sizing of the refrigeration apparatus in cases where the evaporator is installed inside a housing but other components are installed outside the housing of the apparatus by connecting them with the evaporator with a long capillary tube, because then the integral valve mechanisms can be installed together with the components outside the housing.
The refrigeration apparatus may be provided with a third decompression means adapted to decompress the second part of the refrigerant prior to flowing into the first intercooler; and a second intercooler adapted to perform heat exchange between the second part of the refrigerant prior to flowing into the third decompression means and the refrigerant discharged from the evaporator. This arrangement may increase more than conventional apparatuses enthalpy changes by the evaporator during an early stage of startup.
The refrigeration apparatus may be provided with a third intercooler adapted to perform heat exchange between the first part of the refrigerator which has undergone heat exchange in the first intercooler, and the second part of the refrigerant discharged from the condenser.